1. Field of the Invention
The present invention relates to a magnetic disc file for use as an external store of a computer, a thin film magnetic head for use in said magnetic disc file, and a wafer for making said thin film magnetic head, more particularly to a magnetic disc file, thin film magnetic head and wafer for making said thin film magnetic head, affording a high recording density to the magnetic disc.
2. Description of Related Art
A magnetic disc file with a high speed of writing and reading information has been demanded as external store of a computer in the art. The magnetic disc file has been required to have a greater storing capacity with the amount of information to be computerized being increased. On the other hand, it is very important for sales that the size of the system is smaller. Therefore, it is required that the magnetic disc file should have a greater storing capacity, a smaller size and a higher speed of writing and reading information.
The increase of a recording density on a magnetic disc is clearly a key point for affording a greater storing capacity and smaller size to the magnetic disc.
It was already known that the reduction of the thickness of a recording medium layer is necessary to increase the recording density on the magnetic disc. That is, the magnetic head element is formed on the rear end of a flying slider, which flies with a small space maintained on the magnetic disc in writing or reading information, while a magnetic flux emitted for recording information at the end of the magnetic gap of the magnetic head element is spread in a recording medium in case the recording medium is thick. Therefore, when the recording density is to be increased, the recorded magnetic fluxes are closer to each other and have an adverse effect on each other and, as a result, a signal resolution is poor. Thus, the recording medium is required to be thinner for a high recording density. As a magnetic disc satisfying this requirement there has been a thin film magnetic disc such as a sputtering-type magnetic disc or plating-type magnetic disc. The thickness of magnetic layers of these discs can be controlled to be smaller, because the recording medium is formed by sputtering or plating. Since the recording mediums of these discs are continuous and dense, these discs have such an advantage that a relatively large recording magnetization can be obtained. However, as the recording medium is thinner, it is disadvantageous that the recording magnetization becomes smaller and the S/N is reduced. However, if the distance between the magnetic head element and the recording medium is smaller, this problem will be solved. Furthermore, the distance between the magnetic head element and the recording medium should be smaller for reducing the spread of the magnetic flux emitted at the end of the magnetic gap. Thus, in order to obtain a high recording density, it is essential that the thickness of the recording medium is not only reduced but also the distance between the magnetic head element and the recording medium is reduced, i.e., the flying height of the slider is lowered.
The structure of the slider is shown in FIG. 3. FIG. 3 is a pictorial view of a tapered flat-type magnetic head comprising a slider 12 and a magnetic head element 13. The structure of the slider will be explained below. The surface of the slider facing the magnetic disc is divided by straight or curved contours in a plurality of rails. Between the rails there is formed a predetermined amount of steps (FIG. 3 shows an example of rails divided by straight contours). In this case, the surfaces of the divided rails, i.e., the flying faces 14 have a width determined depending upon the designed flying height of the slider. The lower the flying height, the smaller the width. Furthermore, the forepart of the flying face in respect of the relative movement with the magnetic disc is cut obliquely in a forward direction with a small angle of inclination. This is hereinafter referred to as a "tapered part". A relatively great lifting power for flying is exerted on the inner edge of the tapered part and the rear end of the flying face when the magnetic disc is rotated. Therefore, in the example shown in FIG. 3, a main lifting power for flying is exerted on the two inner edges of the tapered parts and the two rear ends of the flying faces. Thus, the slider is supported at the four corners thereof. This effectively inhibits the slider from rolling and pitching, and thus maintains the slider in a stabilized flying pose. Therefore, the flying face of the slider is normally divided into a plurality of parts, which are located approximately over the entire width of the slider. Furthermore, since a larger amount of the lifting power for flying is exerted on the inner edges of the tapered parts, the forepart of the slider is more strongly pushed up. That is, the rear end of the slider is closest to the magnetic disc. Furthermore, since the end of the magnetic gap of a magnetic head element is formed on the surface of the rear end of the slider in such a manner that the end of the gap is exposed on the same plane as that of the flying faces, the above-mentioned tapered parts of the flying slider allow the end of the magnetic gap to be closest to the magnetic disc.
Now, the "flying height" of the flying slider is defined herein as a distance between the magnetic head element in a flying state and the magnetic disc, for convenience. An actual flying height is different from the set one and varies mainly depending upon parameters as mentioned below. Firstly, such difference in the flying height from the set value is based on a dimensional error in the width of the flying face of slider and an error in the force of a slider-supporting spring, secondly on vibrational fluctuation of the distance between the slider and the disc due to the warping of the disc surface, thirdly on temporal vibration of the slider when the slider rapidly moves from one position to another position (seeking movement) for writing and reading another information, and fourthly on many projections generated but yet retained after the projection-removing process by slightly sliding a so-called vanishing slider on the disc surface. The disc surface is not completely even and has many projections generated thereon. The too much removing with the vanish slider will injure the magnetic disc, so the removing is carried out in an amount varying depending upon the set flying height of the slider. However, all of the projections are not removed and, furthermore, some projections should preferably be retained in order to prevent the adsorption between the slider and the magnetic disc. The top of this projection is closer to the slider. The parameters above cause the flying height of the slider to be changed. In the present state of art that the flying height is as small as about 0.3 .mu.m, it is estimated that the first parameter above contributes to a changed amount of about .+-.10%, the second parameter to a changed amount of about .+-.10%, the third parameter to a changed amount of about .+-.10%, the fourth parameter to a changed amount of about -60% to -70%. The marks "+" and "-" mean the increase and decrease of the flying height, respectively. Therefore, the tolerance width of change in decrease of flying height is only 0 to 10%, almost about 5% in the worst case.
Several hundred sliders as mentioned above are normally made from one piece of wafer at a time. The procedure are briefly shown in FIG. 4, which is a flow sheet of process steps for making a magnetic head. Firstly, (a) all magnetic head elements are formed at a time on one piece of wafer. Then, (b) this wafer is machined to be divided into individual sliders. The cut section of the slider is then subjected to the treatment such as additional machining, ion milling or etching to form a positive-pressure type flying slider (e) having a predetermined flying face. FIG. 4 also shows a negative-pressure type flying slider (d) of which two flying faces are connected at the foreparts thereof.
Various metal oxides may be used for the slider and vary depending upon the objects. A typical example of the metal oxides is Al.sub.2 O.sub.3 which has a low density and a high Young's modulus. That is, a lightweight magnetic head is demanded in view of flying stability. A material having a high Young's modulus is used as a slider in order to reduce an amount of deformation during the processing. Furthermore, spinel type oxides or ZrO.sub.2 materials may be used. That is, at the stage of starting or stopping the rotation of the magnetic disc, a transitionally sliding mechanism (CSS mechanism) is generally employed to work on the magnetic head and magnetic disc. Therefore, the sliding with the magnetic head is mainly contributed to by the slider of the magnetic head. Thus, as a slider material is desired a material having a low hardness not to injure the magnetic disc by sliding.
As mentioned above, various metal oxides may be used. The reasons for choosing the oxides are that such materials are inexpensive and sintering can relatively easily be carried out. Furthermore, the process steps of making a thin film magnetic head is complex, and the magnetic head is an article requiring a high precision and mass productivity. Particularly, when the wafer (substrate) undergoes damages such as chipping or dimensional deviation in finally machining a predetermined shape of slider, it is a problem that there is a large amount of loss, because the wafer was already made through the complex process steps. On the other hand, if a wafer is carefully machined so that such damages are to be reduced, it is impossible that a large amount of sliders are made within a given period of time. Therefore, a substrate which has a less amount of chippings and can be made with a high precision has been required.
Many proposals have been made to improve the above-mentioned problems, particularly on Al.sub.2 O.sub.3 substrates. For example, it is disclosed in Japanese Patent KOKAI (Laid-Open) No. 61-158862. In some of the proposals, TiC and a small amount of oxides or metals are added to Al.sub.2 O.sub.3, in order to improve the machinability.
Considering the future high-density magnetic recording disc file, an area recording density is required to be 100 Mb/in.sup.2 or more. In order to obtain such a recording density, the flying height of the slider is required to be set to 0.05 to 0.15 .mu.m. The flying height in this range is also effective to a negative-pressure type slider or perpendicular magnetic recording. Additional problems which have been found in prior art wafers or sliders are that there occurs such a phenomenon that the prior art sliders are deformed when machined and the edge of the rear end of the flying face of the slider goes down below the end of the magnetic head element. The thus deformed slider in a flying state is schematically shown in FIG. 5. Since the slider is deformed, it is seen that the edge of the rear end of the flying face of the slider is below the end of the magnetic head element The amount 19 of a slider 12 deformed is herein defined as being a distance from a base line connecting the two edges projected on the flying face 14 in the largest amounts to each other to the end of the magnetic head element 13 for writing and reading information. Such prior art sliders are not applicable to the case wherein the flying height is lower. In cutting the wafer, the cutting resistance of a grinder at the end thereof is not sufficiently reduced. Therefore, the cut section is curved by warping of the grinder, so that the slider is deformed. Furthermore, the cut section has a machining residual stress retained thereon due to the machining resistance. This stress can be reduced by polishing, but as shown in FIG. 3, there is a difference in the conditions of the machined slider such as the shape and surface finish thereof between the flying face side and the opposite side of the slider. Therefore, there is a difference in the amount of the machining residual stress between the two face sides. This causes the deformation of the slider. The amount of deformation is about 0.01 to 0.02 .mu.m in prior art. There is a tendency that the smaller the Young's modulus of the slider, the larger the amount. However, this was not a problem in prior art. If the flying height is required to be in a small range of 0.05 to 0.15 .mu.m, however, this amount cannot be ignored. As already mentioned, the flying height of the slider, i.e., the distance between the magnetic head element and the magnetic disc surface may be changed. In the worst case, the actual flying height may be about 5% of the set value without considering the deformation of the slider at the rear end thereof. Thus, the deformed amount as shown above is at least 7% to 20% of the set flying height of 0.05 to 0.15 .mu.m, which is clearly beyond 5%. Therefore, when the rear end of the slider is deformed, the slider may dangerously impinge on the magnetic disc. However, since a film formed on the magnetic disc is very thin, such impinging must always be avoided so as not to injure the magnetic disc.
Therefore, the deformation of the rear end of the slider as well as the above-mentioned change in the flying height of the slider must be taken into account from now. If the change in the flying height is taken into account, a tolerated deformation of the rear end of the slider is at most about 5% of the set flying height, i.e., about 0.003 to 0.008 .mu.m, which has not yet been obtained by prior art sliders. In order to solve this problem, the structure of the slider may be considered such that the flying face is close to the center of the slider to reduce an influence from the deformation of the slider. In this case, however, the slider is susceptible to rolling during flying and thus has a problem in respect of flying stability.